1. Field of the Invention
The present invention relates broadly to nuclear well logging apparatus and methods, and pertains in particular to improved apparatus and methods for providing, through the combined logging of neutron characteristic data and gamma ray spectroscopy data, additional and more accurate information concerning the location and recoverability of hydrocarbons in subsurface earth formations.
2. The Prior Art
Generally in logging earth foundations to identify oil-bearing or gas-bearing zones, it is important not only to detect the presence of hydrocarbons but also to determine their relative abundance (saturation) and ease of recovery. In this connection, it is desirable that information be obtained regarding such formation parameters as lithology, shaliness, porosity and salinity, as information of this nature is useful in making quantitative evaluations of hydrocarbon and water saturation levels as well as bearing importantly on the productivity of any given formation.
One form of nuclear well logging intended to afford the foregoing information is described in U.S. Pat. No. 3,521,064, granted July 21, 1970 to James H. Moran et al. and assigned to the assignee of the present application. In the Moran et al. system, a detected gamma ray energy spectrum, illustratively the thermal neutron capture gamma ray spectrum, of an earth formation of unknown composition is analyzed by matching it with a composite spectrum made up of weighted spectra of known constituent materials. By comparing the magnitude of the detected gamma ray energy spectrum at a large number of separate points or energy levels with the magnitude of the composite spectrum, to obtain the best possible match, an accurate analysis of the formation composition can be obtained. Through appropriate selection of the constituent spectra, spectroscopy outputs representative of formation properties of interest, such as porosity, shaliness, salinity, lithology, and the like, may also be derived. Prior systems for implementing the Moran et al. concepts, however, have been subject to perturbational effects and statistical uncertainties which have detracted from their usefulness in some circumstances.
Another form of through-casing nuclear well logging involves the determination of one or more selected neutron characteristics of a formation, including, for example, the thermal neutron lifetime or decay time (.tau.), the macroscopic absorption (capture) cross section (.SIGMA.), and the neutron slowing down time. Systems useful in deriving measurements of these characteristics are described in U.S. Pat. No. 3,566,116 to William B. Nelligan, dated Feb. 23, 1971, U.S. Pat. No. 3,662,179 to Arthur H. Frentrop et al., dated May 9, 1972, and in U.S. application Ser. No. 356,151, filed May 1, 1973 by William B. Nelligan, all of which are assigned to the assignee of this application. Much valuable information, useful especially in distinguishing between salt water and oil and detecting changes in water saturation, is provided by these systems. Interpretation of neutron characteristic logs, e.g., the .tau. and .SIGMA. logs, is however improved by reliable and correlatable data of formation lithology, porosity and shaliness. This is particularly true in low salinity formations, where the .tau. and .SIGMA. logs are less dependable, and where different formations evidence similar .tau. and .SIGMA. values.
Although J. Tittman, in U.S. Pat. No. 3,413,471, dated Nov. 26, 1968 and commonly owned with the instant application, recognizes in general that useful correlations exist between thermal neutron lifetime logs and certain gamma ray spectroscopic logging data, there has heretofore been no integration of neutron characteristic logging and gamma ray spectroscopy logging functions in a manner to provide in a single system sufficient information respecting the various formation parameters of interest to enable full and accurate evaluation of hydrocarbon zones.